1. Field of the Invention
The present invention relates to a height sensor and an air cushion and, more particularly, to a height sensor such as a vehicle height sensor, which is intended to optically sense the height of an automobile or other vehicle, and to an air cushion which contains this height sensor.
2. Description of the Related Art
Recently, there has been an increasing number of vehicles, each of which is equipped with an air cushion in place of a conventional coil spring or leaf spring, in order to simultaneously improve both the comfort of ride and the operating performance in an automobile, which are reciprocal performances.
In the above-mentioned type of vehicle equipped with an air cushion, a vehicle height sensor is mounted inside a suspension system or air cushion so that the height of the vehicle can always be kept constant in accordance with the detected vehicle-height value of the vehicle height sensor. This makes it possible to maintain a constant attitude of the vehicle at all times irrespective of the number of passengers or the amount of load, thereby enhancing both the comfort of ride and operating performance.
The sensor used for controlling the vehicle height mainly includes an optical encoder type sensor or a magnetic encoder type sensor, each of which can be mounted between the vehicle body and a lower spring member of the suspension system. In this way, the sensors are put to practical use. In addition, a magnetic encoder type sensor contained inside the air cushion is also used.
In large-sized vehicles such as a truck, a bus, or the like, a mechanical control valve, which is constructed in such a manner as to detect variations in vehicle height by using a mechanical link and thereby enabling the supply and discharge of the air in the air cushion in accordance with the vehicle height, is mainly used.
The most sensors which have been put to practical use cannot be mounted inside the air cushion. Thus, they are equipped outside the same. For this reason, it is necessary to use a solid casing for guarding the sensor against stones, mud, or water kicked up from a road surface. Also, on account of being externally equipped, a number of steps are required for mounting of the sensor to the vehicle and for adjustment thereof.
In order to solve the above-mentioned problems, a magnetic type sensor, an ultrasonic type sensor, an optical reflection type sensor, etc. which are containable inside the air cushion have been proposed.
However, the magnetic type sensor imposes a limitation upon the stroke of the suspension because of the structure of an air cushion. On the other hand, the ultrasonic sensor has a problem in that the cost is high. Further, most of the optical reflection type sensors are indeed advantageous in that these drawbacks are small in number. However, another problem arises in that if a light reflection plate is dirty causing light reflectance to greatly vary, the measured results also vary. Thus, precision is not ensured.
In order to solve this problem, Japanese Patent Application Laid-Open No. 64-79685 discloses a method of measuring the distance to an object to be sensed in which the influence of the variation in reflectance is eliminated by causing emission of light rays from two kinds of light sources differring from each other in regard to the light emission area, causing reception of the light rays reflected from a reflection plate, separating these light rays from each other and comparing one of them with the other.
However, since this technology uses a plane light-emission element, the dimension of the light-emitting and light-receiving portions of the sensor become large. This raises various problems including limitation for a mountable air cushion, high cost, etc. And because of the peculiar characteristics of this technology, the distance corresponding to the suspension stroke would make accurate measurement difficult.